The ability to scan laser beams by way of reflection from a tiltable mirror serves many purposes in optics. The most general of tilt scanners provide two-dimensional angle scanning from a single reflective surface and are fully programmable. In the visible-light regime, this is typically achieved by using a piezo tilt stage. Piezo tilters, however, have rather a small angle scanning range and require expensive high-voltage drive electronics.
For example, a tilting mirror arrangement used for scanning is described in U.S. Pat. No. 4,708,420 to Liddiard. For this device, a scanning mirror is connected via flexible joints to piezoceramic drive elements that are arranged parallel to the mirror surface. This scanning device is complex and the scanning mirror arrangement is very large so that the piezoceramic drive elements can tilt the mirror through a large angle. This results from the small deflection of the piezoceramic drive elements, which is proportional to the length of these elements. This arrangement is thus not suited to tilt small mirrors through a large angular range when the drive mechanism behind the tilting mirror is to be limited to the dimensions of the mirror surface.
U.S. Pat. No. 4,383,763 to Hutchings et al. describes a tilting mirror apparatus in which the mirror is mounted on a tilting point and is moved by piezoelectric ceramics. Here also, the dimensions of the mirror have to be very large if the tilting mirror is to be tilted through at least 1 degree.
U.S. Pat. No. 4,660,941 to Hattori et al describes a tilting mirror in which the movement of the tilting mirror is effected by piezoelectric elements which act on the mirror via levers. This arrangement is also not suitable for tilting a small mirror through at least 1 degree.
A piezoelectric beam reflector is described in U.S. Pat. No. 5,170,277 to Bard in which the mirror member is directly attached to the piezoelectric element. This device has the disadvantage that the mirror has no defined pivot point when pivoting. Finally, U.S. Pat. No. 4,691,212 to Solcz et al. describes a piezoelectric beam reflector that is used in a scanning arrangement. The disadvantage of this apparatus is that a given deflection angle cannot be rigidly maintained when the pivot point is to remain stationary. As is apparent, the art is in need of a compact tiltable mirror that has a large scanning angle range.
In a related area, extreme ultraviolet (EUV) lithography is an emerging technology in the microelectronics industry. It is one of the leading candidates for the fabrication of devices with feature sizes of 70 nm and smaller. Synchrotron radiation facilities provide a convenient source of EUV radiation for the development of this technology. Though not under serious consideration for high-volume commercial fabrication applications, synchrotron sources play an extremely important role in the development of EUV lithography technology. Being readily available, highly reliable, and efficient producers of EUV radiation, synchrotron radiation sources are well suited to the development of EUV lithography. These sources are currently used for a variety of at-wavelength EUV metrologies such as reflectometry, interferometry and scatterometry.
In the case of synchrotron radiation sources, there are three types of sources: bending magnets, wigglers, and undulators. In bending magnet sources, the electrons are deflected by a bending magnet and photon radiation is emitted. Wiggler sources comprise a so-called wiggler for the deflection of the electron or of an electron beam. The wiggler includes a multiple number of alternating poled pairs of magnets arranged in a series. When an electron passes through a wiggler, the electron is subjected to a periodic, vertical magnetic field; the electron oscillates correspondingly in the horizontal plane. Wigglers are further characterized by the fact that no coherency effects occur. The synchrotron radiation produced by a wiggler is similar to that of a bending magnet and radiates in a horizontal steradian. In contrast to the bending magnet, it has a flow that is reinforced by the number of poles of the wiggler.
Finally, in the case of undulator sources, the electrons in the undulator are subjected to a magnetic field with shorter periods and a smaller magnetic field of the deflection pole than in the case of the wiggler, so that interference effects of synchrotron radiation occur. Due to the interference effects, the synchrotron radiation has a discontinuous spectrum and radiates both horizontally and vertically in a small steradian element, i.e., the radiation is strongly directed.
In lithographic applications, the partial coherence of the illumination (sigma) is often defined as the ratio of the illumination angular range to the numerical aperture of the imaging (projection optical) system. The illumination angular range is also referred to as the divergence of the source. Undulator radiation is much like a laser source in that it produces highly-coherent light of very low divergence. A typical EUV undulator beamline produces a sigma of less than 0.1 whereas lithographic applications nominally require a sigma of 0.5 or higher. Although less coherent than undulator radiation, bending magnet radiation is also typically too coherent to be directly used for lithography.
As EUV lithography technology matures, more lithographic printing stations will be required for resist and process development. Proliferation of these systems has been slowed by the lack of reliable and cost-effective EUV sources. It would be greatly desirable to alleviate the dearth of EUV sources for lithographic process development applications in the form of small-field static microsteppers through the use synchrotron radiation. The relatively high coherence of these sources, however, has precluded them from being used more extensively for actual lithography studies. Relevant process development applications require much more incoherence than is inherently provided by synchrotron sources. This is especially true of undulator sources that otherwise would be highly desirable for their large EUV power capabilities.
A new coherence controlling illuminator that is described in U.S. Pat. No. 6,798,494 to Naulleau addressed some of these problems. This illuminator allows the effective coherence of a synchrotron beamline to be tailored to photolithography applications by using an angular scanning element and a stationary low-cost spherical mirror to re-image the scanning mirror to the reticle plane of the lithographic optic. One significant advantage of this illumination system is that it enables the generation of arbitrary divergence patterns by way of controlling the particular scan configuration. This is of great importance for lithographic process development systems as it enables a single illumination system and source to model a wide variety of divergence patterns that might be generated by the variety of commercial sources and illuminators under development. Hence one process development tool would enable a large number commercial-style tools to be simulated in terms of illumination divergence characteristics greatly increasing the utility of the process development tool.
By design the Naulleau illuminator enables in si tu arbitrary control of the illumination coherence properties (or pupil fill), however, achieving arbitrary and switchable control of the pupil fill requires specialized electronics. In early implementations of the scanning illuminator, conventional function generators were used to generate the scan signals but this made it very difficult to change illumination properties and to achieve complicated fill patterns. What is needed are fully in situ programmable and rapidly switchable drive electronics meeting the requirements of a lithographic process development tool.